Tightening belt - Call me Stupid!

I have a 1987 260E. The serpentine belt looks to be in great shape, but it seems loose while the engine is not running.

I am a decent mechanic, but the MB is not familiar. I found a bolt "thing" that would appear to tighten the belt, but it is maxed out (totally unscrewed).

I have also replaced that shock-absorber thing referred to as the belt tensioner. Further underscoring my ignorance, I don't see how it tensions *anything*.

Questions:
Is there a lock bolt that I am not seeing? Could the belt be stretched but show no visible wear?
There is a ruler-looking gradation just below the water pump. What is that?

Thanks, in advance.

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-Steve